How to Avoid Naming Conflicts in SQL GROUP BY and ORDER BY Clauses?

Tips to avoid naming conflicts in SQL GROUP BY and ORDER BY clauses

In SQL queries involving the combination and sorting of multiple columns, the column names used in the GROUP BY and ORDER BY clauses need to be carefully considered because there may be naming differences between the output column names and the source column names in the SQL language conflict.

For example, the goal of a query is to group data by hardware model (name), attempt type (type), and binary result (result simplified to 0 or 1). However, the desired output is to display only one row for each model with a unique combination of type and case.

The problem arises from using the source column name GROUP BY in both the CASE and result expressions. The SQL standard states that in this case, GROUP BY interprets result as a source column name and ORDER BY as an output column name.

In order to resolve this conflict and get the expected output, there are several ways:

1. Use column alias : Add an alias to the CASE expression, making sure it is different from any source column names. For example:

...
CASE WHEN attempt.result = 0 THEN 0 ELSE 1 END AS result1
...
GROUP BY model.name, attempt.type, result1
...

2. Use positional reference: Use a number to explicitly reference the position of a column in the SELECT list. For example:

...
GROUP BY 1, 2, 3
...

3. contains constants : Although constant columns (for example, day) do not need to be included in GROUP BY, for the sake of clarity they can be added without affecting the results.

Here is the modified query using location references:

SELECT model.name
     , attempt.type
     , CASE WHEN attempt.result = 0 THEN 0 ELSE 1 END AS result
     , CURRENT_DATE - 1 AS day
     , count(*) AS ct
FROM   attempt
JOIN   prod_hw_id USING (hard_id)
JOIN   model      USING (model_id)
WHERE  ts >= '2013-11-06 00:00:00'  
AND    ts <  '2013-11-07 00:00:00'
GROUP  BY 1, 2, 3
ORDER  BY 1, 2, 3;

By ensuring correct column names and using positional references, queries now aggregate data as expected, providing only one row of results for each unique combination of model, type and result.

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